1. Field of the Invention
The present invention relates to X-ray and gamma-ray shielding glass, as well as a method to produce X-ray and gamma-ray shielding glass.
2. Description of the Related Art
Gamma radiation is understood to be radiation with quantum energies of more than 200 keV, regardless of the nature of their origin. In this general sense, the description differentiates from the X-ray radiation. In contrast, X-ray radiation comprises an energy between 100 eV to 300 keV.
Gamma-ray and X-ray shielding glass with a high lead content have become known from a multitude of applications.
Glasses marketed by the Schott Company, having a composition of 34.3 weight-% SiO2, 5.6 weight-% K2O and 59.6 weight-% PbO that are sold under Schott-Glass 8531 are known in the art. This glass composition can be gathered from “Schott Guide to Glass” Second Edition, Chapman & Hall, London 1996, Table 6.3 page 132-133.
EP 1 939 147 A1 shows a gamma-ray shielding glass plate, wherein the glass plate has a glass composition with a high lead content and is characterized in that—at a thickness of 10 mm or more—it offers total light transmission at a wavelength of 400 nm of 50% or more.
From DE 454 430 a glass composition with more than 50% lead oxide and a maximum of 10% zirconium oxide has become known. It is not described in DE 454 430 whether the percentage values regarding the glass composition refer to mol-% or weight-%. Further, a single embodiment having 30 percent SiO2, 5 percent ZrO2, 60 percent PbO and 5 percent K2O has become known from DE 454 430. It is also not known from DE 454 430 that the glass composition can be converted to a plate having high transparency.
WO 2011/052336 A1 describes a lead-containing glass composition having high lead contents of 75 weight-% to 82 weight -% and zirconium contents between 0.5 weight-% and 5.5 weight-%. The problem also exists in WO 2011/052336 that these types of glasses cannot be converted into high transparency plates.
JP 2001/29444 A specifies materials for a plasma display, including a glass powder of a base glass and of a glass filler. The base glass is a glass having a high lead content with 50-75 weight-% PbO, as is the case also with the glass filler. The lead contents of the glasses that, in JP 2001/29444 A contain ZrO2, are higher than 67 weight-%.
DE 102 03 226 A1 describes an optical glass with 55-88 weight-% PbO and 0-10 weight-% ZrO2. The optical glasses according to DE 102 03 226 A1 relate in particular to optical glasses for projection purposes. Plates manufactured from such glasses are not shown in DE 102 03 226 A1.
One disadvantage of all aforementioned glasses during manufacture of the same is the crystallization tendency of these glasses. Because of this, such glasses cannot be produced for example in a draw process, because they crystallize too quickly for such draw processes. An additional problem may be that with such glasses the transmission of light is strongly limited.
The crystallization rate should generally not exceed a limit of 0.1 μm/min. within a temperature range that between the melting temperature of the glass (in this case the melting temperature of the mixture in the melting chamber) and the temperature at which the viscosity of the glass of 106.5-7.0 dPa·s is limited.
The liquidus temperature is the temperature at which, when exceeded a material is completely melted. In practice, it is the highest temperature above which no more crystals are observed. Regarding the observation of crystals, we refer to the following description. The crystallization rate is herein defined by factor dØ/dt, namely the time derivative of the average diameter of the crystals, measured for a certain temperature. If the crystallization rate is greater than 0.1 μm/min. It is necessary—because of their size—to again polish the crystals mechanically. This improves the surface quality as well as the transmission of the glass plate.
Methods for producing glass plates of this type are for example down-draw or up-draw processes, without being limited thereto.
The glasses can of course also be produced by other methods, for example by a redraw method, a rolling method or smaller sizes also in a press process.
In the up-draw process a thin or flat glass is produced, wherein the glass ribbon is drawn from the bottom upwards, via a draw nozzle and various rolls through a cooling section. Up-draw methods are characterized by excellent surface quality and a wide thickness spectrum of 0.8 mm to 20 mm, for example 0.8 mm to 10 mm.
Alternatively to the up-draw method, a down-draw method may be used. The down-draw method serves to produce thin or thinnest glass. In the down-draw method a glass ribbon is also drawn over several rolls through a cooling section, but it is drawn downwards. In the down-draw method, glass ribbons having a low surface roughness that can be less than 1 nm can be produced, wherein the thickness spectrum generally is between 25 μm to 1.1 mm.
An additional disadvantage of the glasses according to the state of the art is the glasses insufficient hydrolytic resistance, which leads to problems, especially when cleaning the glasses, for example when they are used in the medical field.
What is needed in the art is an X-ray and gamma-ray shielding glass composition that avoids the disadvantages of the current state of the art. Plates produced from such a glass composition should have a high transparency and facilitate shielding of X-rays and gamma-rays. In addition to high transparency, such glasses should also have a high hydrolytic resistance. The glass compositions should moreover have a low crystallization tendency and should be suitable for a draw process, for example an up-draw process.